1. Field of the Invention
This invention relates to printed circuit boards and particularly, to plating holes therein. More particularly this invention relates to the preparation of printed circuit boards with plated-through-holes using photosensitive elements in an additive plating process.
2. Description of the Prior Art
In preparing printed circuit boards conductive holes are introduced through the boards to accommodate insertion and soldering of electrical component leads. Conductive holes are conventionally introduced by drilling or punching holes through a copper clad, rigid board followed by a plating procedure. The holes are usually plated by a copper reduction procedure such as that disclosed in Chapter 5 of "Printed Circuits Handbook" edited by Clyde F. Coombs, Jr., published by McGraw-Hill Book Company, New York, New York, 1967 and in Chapter 6 of "Printed Circuits and Electronics Assemblies" edited by C. R. Draper, published by Robert Draper Ltd., Teddington, 1969. The copper clad board with plated-through-holes can then be processed into printed circuit boards using resists and processes as disclosed in the aforementioned "Printed Circuits Handbook" or in any one of a number of U.S. Pat. Nos. e.g., 3,469,982, 3,526,504, 3,547,730, 3,622,334, and 3,837,860. A disadvantage of the conventional copper reduction procedure for plating holes is the waste of expensive catalyst which adheres to the copper cladding as well as the holes, resulting in superfluous overplating of the copper cladding.
Printed circuits can also be prepared by depositing copper conductor patterns directly on insulating substrates by processes such as those disclosed in the following U.S. Pat. Nos. e.g., 3,060,024, 3,146,125, 3,259,559, 3,391,455, 3,506,482, 3,562,038, 3,628,999 and 3,791,858. In preparing printed circuits with plated-through-holes using electroless plating, cleaning procedures are frequently needed after activation of holes and circuit lines in order to remove unwanted catalyst from non-circuit areas before electroless plating. In addition to the cost of the cleaning procedures, expensive catalyst is wasted in the non-circuit areas.